The present invention relates to an improved method of melting charge blanks of electrically conductive, fusible material, and more particularly to an improved method of melting charge blanks by induction heating.
Induction casting equipment is frequently used, in particular for melting small amounts of metal, whether in the form of an alloy or in the form of a pure metal. Such equipment is used more particularly, for example, in connection with dentistry and dental technology, in the jewelry industry and in industrial fine or small-scale casting operations, and in fields of similar kinds. Hitherto, the material to be melted, whether an alloy or a pure metal, has been introduced into a melting pot or crucible in the form of granulated material of shots, bar portions, blocks or cut portions of various shapes and weights. Generally, the governing factor which determines the shapes and sizes of the pieces of material forming the charge to be melted is only the geometry of the crucible, that is to say, the size of the material introduced into the crucible is such that it can be easily disposed therein.
However, this mode of operation suffers from a number of disadvantages, more particularly, for example, pieces of charge material of different shapes and geometries form different induction circuits when subjected to the electromagnetic field of the induction coil used for heating the charge in the crucible and result in high heat radiation losses. Such variations in the induction circuits produced in the material to be melted result in different heating speeds and currents at the surface of the charge material. This phenomenon frequently results in inappropriate or defective processing of the material in the crucible. For example, the material may be overheated, i.e. higher melting temperatures, or may be heated for an excessively long period of time, i.e. higher than necessary energy requirements. Particulary when dealing with alloys, overheating or heating for an excessively long period may have the result that the properties of the material which are required or desired in the finished item and which are to be found in the alloy used as the starting material are detrimentally affected, thus reducing or even destroying the value of the item produced from the processed metal. In this respect, it should be noted that the times required for heating the metal in the crucible to provide for processing thereof may be relatively long in some situations, varying in dependence on the configuration of the pieces of charge material put into the crucible. In addition, the amount of gas which is absorbed and given off by the charge blanks to be melted may vary depending on the configuration of the blank used.
Another problem with previous processes of melting a blank in the above discussed manner is that it is often difficult to accurately determine the proper moment for the pouring or casting operation. If however that moment is not ascertained with a sufficient degree of accuracy, the material in the crucible may be overheated when it is poured or cast, or if the material in the crucible or pot has not yet reached the appropriate temperature, it is possible that particles of pure metal or alloy which have not yet been properly melted or fused in the crucible may be entrained with the flow of molten material as it is poured from the crucible. It will be appreciated that such unmolten or incompletely molten particles will detrimentally affect the quality of the item produced in the casting operation, for example, by forming defects, holes or shrinkage cavities, or porosity in the casting.